1. Field of the Invention
The present invention relates to an electroconductive conjugate fiber. More particularly, the present invention relates to an electroconductive conjugate fiber having an excellent and durable electroconductivity when practically used, a high whiteness and a superior processability, and capable of being produced with an enhanced and stabilized fiber-formability.
2. Description of the Related Art
It is well known that synthetic fibers, for example, polyester fibers, and polyamide fibers exhibit a poor electroconductivity and thus easily generate static electricity by rubbing them. The static electricity charged on the fibers causes various disadvantages, for example, an undesirable adhesion of dust thereto and electric discharge therefrom.
To remove these disadvantages, there have been many attempts to blend electroconductive fibers comprising a white or colorless electroconductive material contained in a fiber-forming polymeric matrix with the non-electroconductive synthetic fibers. Among them, a noticeable attempt is to utilize electroconductive particles comprising white or colorless fine inorganic particles and electroconductive coating layers formed on the particles and comprising, as a principal component, tin oxides.
For example, Japanese Examined Patent Publication (Kokoku) No. 58-39,175 discloses an antistatic polymer composition comprising a matrix consisting of melt-formable synthetic polymeric material and 3 to 20% by weight of fine titanium dioxide particles surface-coated with tin (IV) dioxide (stannic oxide) and dispersed in the polymeric matrix.
In the tin (IV) dioxide-coated titanium dioxide particles, however, the surface-coating layer formed from the tin (IV) dioxide alone is unsatisfactory in that it cannot fully enhance the electroconductivity of the titanium dioxide particles. Therefore, to obtain synthetic fibers having a satisfactory electroconductivity, a specific doping agent is necessarily added to the tin (IV) dioxide coating layers.
Japanese Examined Patent Publication (Kokoku) Nos. 62-29526 and 1-22,265 and Japanese Unexamined Patent Publication (Kokai) Nos. 2-289,108 and 5-51,811 disclose electroconductive conjugate fibers wherein fine electroconductive particles comprising titanium dioxide core particles and electroconductive coating layers formed on the core particle surfaces and comprising a metal oxide and a doping agent, are dispersed in electroconductive segments of the conjugate fibers. In these publications, the electroconductive coating layers are formed from zinc oxide doped with a doping agent consisting of aluminum oxide, or tin (IV) dioxide doped with a doping agent consisting of antimony oxides. These conventional electroconductive particles are unsatisfactory with respect to the whiteness and electroconductivity of the resultant conjugate fibers. Accordingly, in practical production of synthetic fibers having a satisfactory electroconductivity, sometimes the electroconductive particles are permitted to be decreased in terms of the whiteness thereof.
Japanese Examined Patent Publication No. 62-29526 also discloses electroconductive conjugate fibers formed from a thermoplastic polymeric material containing titanium dioxide particles surface-coated with an electroconductive material and a fiber-forming polymeric material. This Japanese publication states that when the conjugate fibers are heat-treated after a fiber-forming step and a drawing step, the electroconductive structure of the fibers is further developed so as to increase the electroconductivity of the fibers. However, it was found that when the size of the electroconductive particles is made small to enhance he fiber-formability (spinability) of the thermoplastic polymeric material, it is necessary to increase the amount of the particles in the thermoplastic polymeric material to obtain a satisfactory electroconductivity of the particle-dispersed polymeric material, the increased amount of the particle causes the particle-dispersed polymeric material to exhibit an undesirably increased melt viscosity thereof and thus an increased difficulty for the fiber-formation from the polymeric material, and the formation of the electroconductive structure by the electroconductive particles becomes unstable and thus the electroconductive performance of the resultant fibers becomes uneven.
Japanese Unexamined Patent Publication (Kokai) No. 4-153305 discloses an electroconductive fiber containing electroconductive particles made from indium oxides. The concretely disclosed electroconductive particles of the Japanese publication are made from indium oxides doped with a tin oxide doping agent. These particles have a light yellowish color and exhibit a significantly high agglomerating property. Therefore, it is difficult to evenly disperse the electroconductive particles in the thermoplastic polymeric material and to form the material into fibers with a satisfactory process stability.
Japanese Unexamined Patent Publication No. 2-307991 discloses a process for producing electroconductive fibers containing electroconductive metal oxide whiskers in place of the electroconductive particles. The whiskers effectively decrease the necessary amount of the electroconductive material. However, the whiskers are disadvantageous in that when the whiskers are mixed into the polymeric material, air bubbles are easily introduced into the polymeric material, and that it is very difficult to uniformly mix the whiskers into the polymeric material and to form the whisker-containing polymeric material into fibers with a satisfactory process stability.
It is well known that when an inorganic filler is mixed into a polymeric material, the surfaces of the filler particles are treated with a coupling agent to enhance the dispersing property of the filler particles and to improve the adhering property of the filler particles to the polymeric material. However, the inventors of the present invention have found that while the conventional coupling agent is contributory to enhancing the dispersing property of the filler particles and the fiber-forming stability of the filler-containing polymeric material, and the resultant fibers exhibit an unsatisfactory electroconductivity and durability in practical use.
Japanese Unexamined Patent Publication (Kokai) No. 60-110,920 discloses an electroconductive conjugate fiber having an electroconductive segment in which an electroconductive substance comprising a metal oxide core particle and an electroconductive coating layer formed on the core particle surface is dispersed in a thermoplastic polymeric material. This Japanese publication discloses various types of electroconductive particles each having an inorganic core particle made from a member selected from tin oxides, zinc oxide, titanium dioxide, magnesium oxide, silicon oxide, and aluminum oxide, and an electroconductive surface-coating layer formed from a member selected from tin oxides, zinc oxide, copper oxides, indium oxides, zirconium oxides, and tungsten oxides. Also, the Japanese publication teaches to add a small amount of a secondary component to the electroconductive surface-coating layer to enhance the electroconductivity of the surface coating layer. The electroconductive particles concretely disclosed in the Japanese publication consisted of titanium dioxide core particles and electroconductive surface-coating layers formed on the core particles and comprising tin oxides doped with a small amount of antimony oxides. The resultant electroconductive particles have a light bluish grey color close to white. These electroconductive particles are still unsatisfactory to obtain electroconductive fibers having both high whiteness and the satisfactory electroconductivity.